This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The aim of this study is to determine the effects of physiological stress on coronary blood flow in humans under conditions of static hand grip exercise, voluntary and involuntary muscle contraction, hypoxia/hyperoxia/hypercapnia, lower body negative pressure and cold pressor test. Coronary flow will be measured with echocardiography.